1. Field of the Invention
This invention relates to a spark plug.
2. Description of the Related Art
A spark plug used for ignition of an internal engine of such as automobiles generally comprises a metal shell to which a ground electrode is fixed, an insulator made of alumina ceramics, and a center electrode which is disposed inside the insulator. The insulator projects from the rear opening of the metal shell in the axial direction. A terminal metal fixture is inserted into the projecting part of the insulator and is connected to the center electrode via a conductive glass seal layer which is formed by a glass sealing procedure or a resistor. A high voltage is applied to the terminal metal fixture to cause a spark over the gap between the ground electrode and the center electrode.
Under some combined conditions, for example, at an increased spark plug temperature and an increased environmental humidity, it may happen that high voltage application fails to cause a spark over the gap but, instead, a discharge called as a flashover occurs between the terminal metal fixture and the metal shell, going around the projecting insulator. Primarily for the purpose of avoiding flashover, most of commonly used spark plugs have a glaze layer on the surface of the insulator. The glaze layer also serves to smoothen the insulator surface thereby preventing contamination and to enhance the chemical or mechanical strength of the insulator.
In the case of the alumina insulator for the spark plug, such a glaze of lead silicate glass has conventionally been used where silicate glass is mixed with a relatively large amount of PbO to lower a softening point. In recent years, however, with a globally increasing concern about environmental conservation, glazes containing Pb have been losing acceptance. In the automobile industry, for instance, where spark plugs find a huge demand, it has been a subject of study to phase out Pb glazes in a future, taking into consideration the adverse influences of waste spark plugs on the environment.
Leadless borosilicate glass- or alkaline borosilicate glass-based glazes have been studied as substitutes for the conventional Pb glazes, but they inevitably have inconveniences such as a high glass viscosity or an insufficient insulation resistance. In particular, since the glazes for spark plugs are used attaching to engines, they are apt to rise in temperature than cases of general insulating porcelains (maximum: about 200xc2x0 C.). Further, in recent years the voltage applied to spark plugs has been increasing together with advancing performance of engines. For these, the glaze for this use has been required to have insulation performance withstanding severer conditions of use. Practically, for restraining flashover at heightened temperatures, requisite is such a glaze having excellent insulating properties.
In conventional leadless glazes for spark plugs, in order that a melting point is checked from rising by exclusion of a lead component, an alkaline metal component has been compounded. The alkaline metal component is useful for securing fluidity when baking the glaze. But it decreases the insulation resistance of the glaze as increasing of the containing amount, and also has an aspect to easily spoil the anti-flashover, it is desirable that the alkaline metal component has a necessarily least amount.
Accordingly, the conventional leadless glaze is apt to be short in the containing amount of the alkaline metal component, and the glass viscosity easily becomes high at high temperatures (when the glaze melts) in comparison with a Pb glaze, and after baking the glaze, pinholes or glaze crimping appear in an external appearance.
It is an object of the invention to provide such a spark plug having a glaze layer which has a reduced Pb content, is low in the glass viscosity at high temperatures, and exhibits high insulation properties.